Optical integrated circuits (OICs) come in many forms such as 1×N optical splitters, optical switches, wavelength division multiplexers (WDMs), demultiplexers, optical add/drop multiplexers (OADMs), and the like. Such OICs are employed in constructing optical networks in which light signals are transmitted between optical devices for carrying data and other information. For instance, traditional signal exchanges within telecommunications networks and data communications networks using transmission of electrical signals via electrically conductive lines are being replaced with optical fibers and circuits through which optical (e.g., light) signals are transmitted. Such optical signals may carry data or other information through modulation techniques, for transmission of such information through an optical network. Optical circuits allow branching, coupling, switching, separating, multiplexing and demultiplexing of optical signals without intermediate transformation between optical and electrical media.
Such optical circuits include planar lightwave circuits (PLCs) having optical waveguides on flat substrates, which can be used for routing optical signals from one of a number of input optical fibers to any one of a number of output optical fibers or optical circuitry. PLCs make it possible to achieve higher densities, greater production volume and more diverse functions than are available with fiber components through employment of manufacturing techniques typically associated with the semiconductor industry. For instance, PLCs typically comprise optical paths known as waveguides formed on a silicon wafer substrate using lithographic processing, wherein the waveguides are made from transmissive media including lithium niobate (LiNbO3) or other inorganic crystals, silica, glass, thermo-optic polymers, electro-optic polymers, and semiconductors such as indium phosphide (InP), which have a higher index of refraction than the chip substrate or the outlying cladding layers in order to guide light along the optical path. By using advanced photolithographic and other processes, PLCs are fashioned to integrate multiple components and functionalities into a single optical chip.
One important application of PLCs and OICs generally involves wavelength-division multiplexing (WDM) including dense wavelength-division multiplexing (DWDM). DWDM allows optical signals of different wavelengths, each carrying separate information, to be transmitted via a single optical channel or fiber in an optical network. For example, early systems provided four different wavelengths separated by 400 GHz, wherein each wavelength transferred data at 2.5 Gbits per second. Current multiplexed optical systems employ as many as 80 wavelengths, and systems are contemplated having more than 160 wavelength channels with 50 GHz spacing, carrying data at 10 Gbits per second in each channel.
In order to provide advanced multiplexing and demultiplexing (e.g., DWDM) and other functions in such networks, arrayed-waveguide gratings (AWGs) have been developed in the form of PLCs. Existing AWGs can provide multiplexing or demultiplexing of as many as 80 channels or wavelengths spaced as closely as 50 GHz, and AWGs are contemplated to accommodate 128 wavelengths spaced at 25 GHz. As illustrated in FIG. 1, a conventional demultiplexing AWG 2 includes a base 4, such as a silicon substrate, with a single input port 6, and multiple output ports 8. Multiple wavelength light is received at the input port 6 (e.g., from an optical fiber in a network, not shown) and provided to an input lens 10 via an input optical path or waveguide 12 in the substrate base 4.
The input lens 10 spreads the multiple wavelength light into an array of waveguides 14, sometimes referred to as arrayed-waveguide grating arms. Each of the waveguides or arms 14 has a different optical path length from the input lens 10 to an output lens 16, resulting in a different phase tilt at the input to the lens 16 depending on wavelength. This phase tilt, in turn, affects how the light recombines in the output lens 16 through constructive interference. The lens 16 thus provides different wavelengths at the output ports 8 via individual output waveguides 18, whereby the AWG 2 can be employed in demultiplexing light signals entering the input port 6 into two or more demultiplexed signals at the output port 8. The AWG 2 can alternatively be used to multiplex light signals from the ports 8 into a multiplexed signal having two or more wavelength components at the port 6.
A problem with the AWG 2 of FIG. 1 is polarization dependence of the waveguides 14, caused by waveguide birefringence. Waveguide birefringence is experienced in varying degrees with waveguides fabricated from the above-mentioned materials. For example, where the waveguides 14 are formed by depositing a glass film on a silicon substrate, the difference in thermal expansion coefficient between the glass film and the silicon substrate base 4 causes stress applied on the waveguides 14 in a direction parallel to the surface to be different from that in a perpendicular direction.
Waveguide birefringence results, wherein the refractive index of the waveguides 14 in the direction parallel to the substrate surface becomes different from that in the perpendicular direction. The birefringence, in turn, causes polarization dependence in the waveguides 14, where the optical path length difference (e.g., between adjacent waveguides 14) changes depending on the polarizing direction of light. In this situation, shifts occur between the transverse electric (TE) and transverse magnetic (TM) mode peaks, where the shift changes according to polarization. Consequently, the device characteristics change in accordance with the polarized state of the light provided to the device 2. For instance, the peak coupling in a particular channel or waveguide 14 can vary according to the polarities of the various wavelength components, causing polarization dependent wavelength (PDW) shift.
Referring to FIGS. 2-4, this polarization sensitivity or dependence in AWGs and other dispersive components has been heretofore addressed by bisecting the waveguides 14 and placing a polarization swapping device or waveplate, such as a half-waveplate 20, in a slot 21 between waveguide portions 22 and 24. The waveplate 20 is polarized at a 45 degree angle with respect to the X and Y axes, where the waveguide 14 intersects the slot 21 along the Z axis. Thus located, the waveplate 20 reduces or eliminates polarization dependence in the AWG 2. In particular, it has been found that the waveplate 20 causes polarization swapping partway along the optical paths of the bisected waveguides 14, such that any input polarization samples each propagation constant equally and provides essentially no shift in peak-wavelength with changes in input polarization. Thus, the spectrum for the TE and TM modes coincide through the use of the waveplate 20. Conventionally, the waveplate 20 is placed at the precise center of the grating arms or waveguides 14 to eliminate the wavelength shift resulting from birefringence.
Referring now to FIGS. 3 and 4, cutting the groove or slot 21 into the AWG 2 can cause reduced performance of the device, and complex assembly procedures are needed to properly install a polarization swapping waveplate 20 therein. Moreover, it is difficult to monitor and/or adjust or tune the polarization swapping performance of the installed waveplate 20 in the slot during manufacturing. Consequently, there remains a need for better solutions to polarity dependence in optical integrated circuits such as AWGs, which avoid or mitigate the performance reductions and complex assembly steps associated with the convention employment of waveplates in slots in such devices.